1. Field of the Invention
The present invention relates to a semiconductor memory device and a method for fabricating the same, and in particular such that being preferably applicable to those having a charge storage film for storing electric charge.
2. Description of the Related Art
Any semiconductor memory device storing data by storing electric charge has a charge storage film for storing the charge, and writes or erases data by varying threshold voltage (Vth) of the memory cell transistor depending on the amount of charge stored in the charge storage film.
The aforementioned writing or erasure of data in, for example, a SONOS (semiconductor/oxide film/nitride film/oxide film/semiconductor)-type semiconductor memory device is effected by generating a potential difference between a gate electrode (word line) of a selected memory cell and a semiconductor substrate (bit line), and then by injecting hot electrons into the charge storage film, or by injecting holes based on band-to-band tunneling.
Conventional fabrication of the semiconductor memory device has, however, been suffering from a problem that the charge storage film was likely to store unnecessary charge during various process steps in the fabrication. This resulted in generation of error or variation of charge to be stored in the charge storage film when the semiconductor memory device is operated for writing or erasure, which undesirably varied the threshold voltage and prevented the device from being stably operated.
The present invention is accomplished considering the aforementioned problem, and is to provide a semiconductor memory device and a method of fabricating the device, both of which being aimed at avoiding storing of unnecessary charge into the charge storage film, and thus stabilizing the threshold voltage.